Gandevia and team have discovered that when human muscles are completely relaxed, the muscle fibres don't just shorten, but actually become wavy and buckle.

Although this sounds paradoxical, it means that at rest, muscles are under no tension whatsoever.

"Just imagine a coil of rope or wire that had become so low in tension [or slack] that it buckled," says Gandevia.

Gandevia and colleagues recruited 25 adults aged 21 to 86 with no history of musculoskeletal injury for their study.

While the participants lay on a table with their left knee bent, their left ankle was strapped into a footplate.

The footplate moved up and down so it alternately bent and straightened the ankle, forcing the fibres within the muscles to alternately lengthen and shorten.

'Shorter than short'

Ultrasound images were taken to see what was going on in the muscle fibres themselves.

"We were completely surprised at what we saw. We had previously put together some evidence that when muscle fibres are short, they really weren't producing any effective tension - but we never knew they got 'shorter than short' - that they actually buckled," says Gandevia.

"It's dramatic in physiology when you can visualise something on a screen that nobody has ever seen before in human muscle."

Gandevia says his team had been interested in what happens in the passive property of muscles, when there is no contraction.

"These properties are important, because they determine at what angle your joints might sit when you are relaxed," he says.

Implications

The discovery will allow researchers to build more accurate models of muscle function and improve understanding of disorders where the muscles become really short, says Gandevia, including after a stroke, or in multiple sclerosis, where you can't, for example, straighten out your elbow all the way.

Gandevia says the next stage in the research would be to pick some patients who have an abnormal muscle state to see whether the tendency to form this buckling is altered or not.

"It would give us insight into the changes that have taken place in the pathologically affected muscle. It would probably give us some clues as to which part of the muscle have had those changes," he says.

"Now we know about the buckling, it could give you a measurement point because you can take the muscle to a particular length and know it begins to buckle at that length. Then you could follow patients up later, and see whether or not that particular angle [of the limb] at which buckling began had altered."